Surveys over geological structures are generally conducted using seismic data acquisition methods or electromagnetic acquisition methods. Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. Using conventional acquisition techniques, an ocean-going vessel is used to tow one or more acoustic sources and one or more seismic streamer cables through the ocean along predetermined sail lines. A suitable acoustic source is created by the collapsing of an air bubble, and prior art acoustic sources typically comprise compressed air guns for generating acoustic energy in the water called ‘shots’. The basis of marine seismic data acquisition methods is the accurate timing of artificially generated pulses of acoustic wave energy that propagate through the ocean and are reflected at the interfaces between subsurface formations. These reflected pulses which are referred to as “seismic energy” or “seismic signals” (because of the interaction of the acoustic energy with the geological formation) are detected using transducers called hydrophones that transform the seismic energy into electromagnetic signals. Each streamer towed behind the vessel typically supports multiple hydrophones and the data collected by each hydrophone is recorded and processed to provide information about the underlying subsurface geological features. Using conventional acquisition techniques, towing of the streamers is undertaken at a predetermined speed and along predefined parallel and linear sail lines to assist with the collection and processing of the data acquired by the hydrophones.
A portion of the acoustic energy fired from an acoustic source travels downwardly through a body of water towards a subsurface geological and a portion thereof is reflected upward from the subsurface geological formation as a response signal. This response signal is collected at a hydrophone. The amplitude and the time taken for the response signal to be received at the hydrophone are indicative to some degree of the depth of subsurface geological formation. At the time that the data is being collected at the hydrophones, there is no existing knowledge as to the extent in area of the subsurface formation (as defined by its x and y co-ordinates) or the depth z of the subsurface interfaces at which seismic energy is reflected. Mathematical operations based on the acoustic wave equation above are used to “migrate” the signals collected by the hydrophones to their x, y and depth co-ordinates of the subsurface reflection points. All of these “migration” algorithms require stable and consistent spatial sampling of the measured wave field in order to accurately reconstruct the correct position, depth and importantly the amplitude and phase of the signal which may get used later in the upstream flow for hydrocarbon prediction.
The use of one streamer towed along a single linear sail line at a time collects a limited set of what is referred to as a “2-D in-line seismic data”, which is a useful and relatively inexpensive way of conducting a marine seismic survey. When a single streamer is towed along a single sail line, cross-line data is not acquired and the data set has an azimuth of essentially 0±10 degrees which is the industry accepted limited of feather tolerated when acquiring 2D in-line data using the methods of the prior art. These signals received by the hydrophones can be collated together in what is termed a “gather” by collecting the source-hydrophone pairs that share a CMP. The number of source/hydrophone pairs that make up a gather is subsequently termed the “fold” of the gather.
A “3-D seismic data set” is generated when multiple streamers are towed in parallel along a single linear sail line. It is not unusual for the streamers to be spaced up to 100 meters apart and be 6000 meters long. The number of streamers and the size of the area being surveyed determine to a large degree the cost of a seismic survey. The size of the vessels required to tow these long streamers over vast areas of ocean also contribute substantially to the cost of the survey. Due to the total number of sail lines required to build coverage of an area of interest, it is generally cheaper and therefore more desirable to use the prior art 3D acquisition methods than the prior art 2D acquisition methods described above. By way of example, assuming that the area being surveys is 50 km wide and 20 km across and using the 3D streamer array of FIG. 8, the full survey area can be traverse using 80 parallel sail lines at a distance of 250 meters apart. To collect the same density of data using an equivalent prior art 2D seismic acquisition arrangement would require 400 sail lines to be traversed. This gives a cross-line bin dimension of 50 meters.
Using either 2D or 3D surveying, multiple parallel adjacent linear sail lines are traversed so that the traversed ocean surface area overlays the subsurface area of interest. Using the methods of the prior art, the quality of the acquisition of seismic data relies to some extent on the skill of the towing vessel operator to accurately traverse the predefined parallel adjacent linear sail line/s and their ability to ensure that the orientation of each streamer is maintained parallel to and in line with the linear sail lines. When there are multiple streamers as used for 3D seismic acquisition, that task is not only very difficult but is also critical to the quality of the information collected. It is not uncommon to abandon a sail line part way through because the streamers can not be kept parallel to the sail line due to loss or lack of control or strong currents and adverse weather conditions and consequently great expense can be incurred because of delays or the need to redo all or part of a predefined sail line.
Methods have been proposed in which marine seismic data is acquired with the streamer towing vessel following a circular sail line. For example, U.S. Pat. No. 4,486,863 (French) discloses a method wherein a streamer towing ship moves along circular paths with the streamer following this circular path. Each of the circles is offset along an advancing line. The towing ship completes a full circle and then leaves the completed circle tangentially to move on from one circle to the next. However, there is a finite amount of curvature that can be put on a streamer resulting in a large track distance ratio (i.e. a large ratio between the actual distance traversed by the vessel compared with the nominal sail-line distance). It is also very difficult to insert in-fill passes using this style of acquisition. This is a very inefficient way to collect 3D seismic data, and the additional time taken to acquire the data equates to an increase in the cost of the acquisition.
U.S. Pat. No. 4,965,773 discloses a method of gathering and mapping seismic data of a marine region which contains a stationary body comprising the steps of defining a spiral path using a point on the body as the origin of the spiral, and towing a transmitter/receiver streamer along the spiral path to gather seismic data. The method is directed for use in data collection around objects such as small islands, salt fingers present in the substratum of similar point-like structures. In the preferred embodiment, the radial distance between the spiral turns is constant as given by an Archimedean spiral. This is also a very inefficient way to collect 3D seismic data, and the additional time taken to acquire the data equates to an increase in the cost of the acquisition.
US Patent Publication Number 2008/0285381 (Moldoveanu et al) describes towing a seismic spread including a single source and a plurality of streamers with all of the streamers being actively steered to maintain each of the streamers on a generally curved advancing path. The radius of the generally curved advancing path is described as being around 5,500-7,000 m, resulting in a curved path having a circumference of around 34,000-44,000 m. Given an average streamer length of around 6,000 m, it can immediately be seen the length of each streamer covers only a small arc-length of the circular path being traversed. This method of acquisition will inherently have only a small amount of deviation from traditional linear 3D acquisition systems with the added expense of having to actively steer a plurality of streamers to avoid entanglement of the streamers during acquisition.
Methods have also been proposed in which marine seismic data is acquired with the streamer towing vessel following a linear sail line with the source vessel traversing a non-linear path. By way of example, U.S. Pat. No. 3,806,863 (Tilley et al) discloses a system including a streamer vessel traversing a linear sail line with one or more source vessels or “shooting boats” traversing a zigzag course. The shooting boat proceeds along the firing leg from a position near to the base course of the recording boat to its sideways extreme position, or vice versa. The seismic source of one of the shooting boats is fired at a repetitive time interval as that shooting boat traverses a flanking zigzag course line including a firing leg, or segment, oblique to the base course of the recording boat. This method of acquisition inherently demands the use of more than one vessel which can result in unnecessary expense with limited flow-on benefits. More significantly, controlling the zig-zag course of the shooting vessels is very difficult to achieve in practice, making this method prohibitively expensive.
U.S. Pat. No. 3,921,124 (Payton) describes a method of deriving a 3D seismic dataset with a seismic vessel pulling a single streamer which traverses a linear sail line. At the same time, one or more remote controllable mobile seismic sources are actively steered along a periodic path which facilitates the determination of common depth point data along a plurality of lines parallel to the line of survey, thereby producing 3D seismic information. The positions of the sources are systematically controlled using a “controlled paravane”. A seismic source is a large piece of equipment that requires a fair amount of force to deviate. To the best of the knowledge of the present inventors, such a controllable paravane has never actually been designed and employed to date.
There remains a need in the art for an alternative marine seismic data acquisition method and related system that produces a relatively cheap, multi-azimuth seismic dataset.